Electrolytic production of adiponitrile



Jan. 6, 1970 w. J. SLQAN 3,488,267

ELECTROLYTIC PRODUCTION OF ADIPONITRILE Filed June 25, 1963 FIGo\INVENTOR WALTER JOHN SLOAN ATTORNEY United States Patent ABSTRACT OFTHE DISCLOSURE Process for the production of adiponitrile whichcomprises passing a direct electric current through an electrolytecontaining acrylonitrile, a quaternary ammonium compound and water.

This invention relates to the production of adiponitrile fromacrylonitrile. Specifically, this invention relates to the production ofadiponitrile by passing a direct electric current through an electrolytecontaining acrylonitrile using at least one quaternary ammonium compoundin the electrolyte.

It is known that acrylonitrile can be reacted to produce adiponitrile bymeans of potassium amalgam which is produced electrolytically. Theamalgam then reacts chemically with acrylonitrile and hydrochloric acidto form adiponitrile. It is an object of this invention to provide analternative process for the production of adiponitrile which does notrequire the use of amalgam. It is a further object of this invention toprovide a process for the production of adiponitrile at high currentefliciency. Other objects of this invention will be apparent to oneskilled in the art from the remainder of the specification.

Theabove objects are accomplished according to the present invention bypassing a direct electric current through acrylonitrile containing fromabout 1 to about 3.8% by weight water and a quaternary ammoniumcompound. The process of this invention may be carried out with orwithout a diaphragm in the electrolytic cell. If a diaphragm is to beused, it is preferable to employ a cationic permselective membrane (thatis, a membrane that is only cation-permeable). Such diaphragms are knownin the art and arecommercially available. If a cationic permselectivemembrane is used, an anolyte may be employed that contains noacrylonitrile. This can be advantageous for under some conditions theanodic oxidation products tend to cause polymerization of theacrylonitrile. However, a diaphragm is not necessary to the successfuloperation of the process of this invention.

In' the drawings FIGURE 1 shows a cross-sectional view through adiaphragmed electrolytic cell suitable for use in carrying out theprocess of the present invention.

FIGURE 2 shows a cross-sectional view through an electrolytic cellwithout a diaphragm that is suitable for carrying out the process of thepresent invention.

The quaternized ammonium compounds found to be useful as electrolytesfor the process of the present invention have the following generalformula:

where R is an organic radical having 2-8 carbon atoms, and X- is amember of the class consisting of Cl, Br, I, and R 80 where R is amonovalent alkyl group containing between 1 and 6 carbon atoms. Specificexamples of compounds known to be useful are: tetraethyl ammoniumbromide, tetrabutyl ammonium iodide, tetrabutyl ammonium bromide,tetraethyl ammonium ethyl sulfate. In general, quaternized ammoniumcompounds useful in the instant process must be good conductors whendissolved in moist acrylonitrile and be stable at a potential of atleast about 2.0 volts.

3,488,267 Patented Jan. 6, 1970 The process of this invention is carriedout at a cathode current density of between about 50 and about 200amp/ft. and at a measured voltage (cathode to catholyte) of betweenabout 1.2 and 15.0 volts. The process is preferably carried out Withinthe temperature range of 30 to 70 C. because the conductivity of theelectrolyte is greatest at this temperature. The optimum temperature isabout 45 C. The pH of the system is critical only in that the pH shouldbe on the acid side, i.e., less than 6, for higher pH may causepolymerization of the acrylonitrile. The concentration of the quaternaryammonium compound in the moist acrylonitrile can vary over wide limitsdepending on the particular compound employed, but about 430% by weightof the moist acrylonitrile is satisfactory in most cases (concentrationsof about 6 to 15% by weight are preferred).

It is highly desirable, when carrying out the process of this inventionin either an undiaphragmed' cell, or in a diaphragmed cell, to agitatethe portion of the cell in which the acrylonitrile is undergoingconversion. Agitation avoids areas of high pH and reduces the degree ofpolarization, resulting in increased yields. The degree of agitationuseful in the present process varies from a Reynolds number of about12,000 to about 200,000, preferably about 50,000 to about 125,000.

When the process of this invention is carried out in a diaphragmed cell,the diaphragm is preferably a cation permselective diaphragm. Suitableion exchange membranes are commercially available. Useful membranes canbe made by sulfonating a copolymer of styrene and divinyl benzene, andblending the product with polyethylene. The blend is then formed into asheet of about .045 inch thickness. Sulfonated copolymers of styrene anddivinyl benzene may be ground and mixed with sulfonated polyethylenedissolved in a suitable solvent and cemented as a sheet to Dynel (acopolymer of vinyl chloride and acrylonitrile) monofilament fabric. Somemembranes require pre-soaking prior to use. This can be accomplished bytechniques known in the art, for example the membrane is installed inthe cell, and then treated with aqueous H (5%). This treatment avoidsuneven swelling and buckling. The anolyte in the diaphragmed cell is anaqueous acid. Sulfuric acid at concentrations of 25 to 45% is highlysatisfactory. Also dilute hydrochloric acid can be used, whensatisfactory means are available to remove the chlorine. Aqueous organicacids such as ethyl sulfuric acid and p-toluene sulfonic acids are alsosatisfactory.

Cathode materials suitable for the process include lead, platinum,palladium, copper, nickel, chromium on brass, and silver. Platinum,lead, silver and copper are the preferred cathode materials. The anodemay be of any relatively inert conductor; platinum and carbon are highlysatisfactory.

The catholyte, or in the case of an undiaphragmed cell, the electrolyte,may contain (in addition to acrylonitrile, water, and quaternaryammonium compounds) other materials that do not substantially affect thebasic composition. For example, cosolvents for the quaternary ammoniumcompound, may be included. Specifically, dimethyl formamide, methanol,or isopropanol may be added to the catholyte as cosolvents fon theammonium compound.

When carrying out the process of this invention for extended periods oftime, it is usually desirable to peri odically or continuously add acid,such as sulfuric or hydrobromic, to the electrolyte in order to maintainthe pH below 6.

The drawings diagrammatically illustrate alternative apparatus suitablefor carrying out the process. FIG- URE 1 shows an electrolytic celldesignated 1, having a cathode 2 in catholyte 3, and an anode 4 inanolyte 5.

The anolyte and the catholyte are separated by a cation permselectivediaphragm 6. The cathode compartment of the cell contains an agitator 7.Suitable inlet means 8 and outlet means 9 are provided for the cathodecompartment. The anode compartment has inlet means 10 and outlet means11. The cell is also provided with suitable vents 12 and 13 to removethe gaseous electrolytic products.

FIGURE 2 diagrammatically shows an undiaphragmed electrolytic cellsuitable for use in the process of this invention. The cell 1 isprovided with a cathode 2, an anode 4, an agitator 7, an inlet means 8,an outlet means 9, and vents 12 and 13. Additionally, inlet means 14 isprovided to allow the introduction of acid to keep the pH acidic.

In operation either of the cells may be run in a batchwise fashion orcontinuously. That is, electrolyte (moist acrylonitrile and quaternaryammonium compound) may be introduced continuously through inlet 8 andWithdrawn continuously through outlet 9; or the electrolyte may beintroduced through inlet 8, electrolyzed and withdrawn as a batchthrough outlet 9. In either instance, the adiponitrile is separated fromthe acrylonitrile, water, and quaternary ammonium compound by furtherprocessing after removal through outlet 9. I

In the following examples which illusrate the invention, all parts andpercentages are in parts 'by weight unless otherwise stated.

EXAMPLE I Into the cathode compartment of a cell having a diaphragm ofAmberplex C1 (an ion exchange membrane made by blending a sulfonatedcopolymer of styrene and divinyl benzene with polyethylene, and thenforming a sheet of the blend), 350 ml. of a moist acrylonitrile,approximately 3.6% by weight water, containing about 7.5% by weighttetraethyl ammonium bromide was introduced. Into the anode compartments,30% by weight H 50 was simultaneously introduced. The membrane had anarea of approximately 0.024 sq. ft. The membrane had a thickness ofapproximately 0.045 in. The catholyte was vigorously agitated by meansof a stirrer and voltage of approximately 8.4-9.0 volts applied betweena platinum anode and a lead cathode for about 4.8 hours. The cathodecurrent density was about 50 amp/ftP. The cathode to catholyte potentialdrop was measured as 3.2 volts. The temperature was approximately 30 C.The pH varied from 1 to 5. The current efficiency was about 81%calculated with regard to amount of acrylonitrile converted toadiponitrile. 79% of the tetraethyl ammonium bromide was recovered fromthe catholyte.

EXAMPLE II The process of Example I was repeated using a catholytecontaining 26.9% by weight tetraethyl ammonium ethyl sulfate instead ofthe tetraethyl ammonium bromide. The anolyte was an aqueous solution 12%by weight of ethyl sulfate C H SO I-I. A voltage of 8.4-9.4 volts wasapplied between a platinum anode and a lead cathode. The cathode tocatholyte potential drop was measured as 2.4 to 2.9 volts. The pH of thecatholyte was less than 1. The current density at the cathode was 100amp/ sq. ft. The process was carried out using vigorous agitation andthe time was 3.7 hours. The amount of water in the catholyte at thestart of the run was about 3.0% by weight. The current efficiencycalculated with regard to the amount of acrylonitrile converted toadiponitrile was 73.5%.

EXAMPLE III and the cathode was platinum. A 48% solution of hydrobromicacid was periodically added to the electrolyte to maintain the pHbetween 1 and 3.5. The product was recovered and analyzed. The currentefiiciency for the production of adiponitrile was 34%.

EXAMPLE IV In a cell, such as illustrated in FIGURE 2, an electrolyteconsisting of 44.4.parts acrylonitrile, 44.5 parts dimethyl formamide,(a cosolvent for the quaternary ammonium compound), 7.6 partstetraethyl ammonium bromide, and 3.5% water, was electrolyzed with mildagitation at a cathode current density of 65 amp/ft. for 60 minutesusing a platinum cathode and a platinum screen anode. Hydrogen gas wasintroduced in the anode region in an effort to minimize thepolarization. A 48% solution of hydrobromic acid was introducedperiodically into the electrolyte to maintain the pH at about 2.2. Theproduct was removed and analyzed. The current efficiency for theproduction of adiponitrile was 12.5%

EXAMPLE V An electrolyte of 87.2 parts acrylonitrile, 9.8 partstetraethyl ammonium ethyl sulfate, and 3.0 parts water was electrolyzedwith mild agitation in a cell such as that shown in FIGURE 2, at apotential of 9.6 volts, at a cathode current density of 65 amp/ft. for34 minutes. The pH remained about 0; no acid was added. Both the anodeand cathode were platinum. The product was analyzed and the currentefiiciency for the production of adiponitrile was calculated to be 52%.

Two methods were used for product work-up. Method I was used when lowrrielting quaternary ammonium compounds were used, i.e.,- melting pointnot in excess of about 200 C. In both methods, the content ofadiponitrile was confirmed by chromatographic analysis.

Method I 1.0 N NaOH was added to the product of the electrolytic processto raise the solution pH to 5. The material was distilled and an organiccut (primarily acrylonitrile) was collected until a head temperature of99 C. was reached. The cooled pot contents were then filtered and theresidue weighed. The water was then removed from the filtrate bydistillation at atmospheric pressure until a head temperature of 110 C.was reached, followed by distillation at 40 mm. Hg. To remove finaltraces of water, 300 ml. of chloroform was added to the pot and awaterchloroform azeotrope taken ofi until the condensate appeared clear.The pot contents were cooled and filtered to remove insoluble sodiumsalts. Most of the chloroform was removed from the filtrate over a steambath at atmospheric pressure, the remaining traces being taken off undervacuum. About 300 m1. of benzene was then added to the pot and thedistillate removed until a head temperature of C. was reached. The potcontents were then cooled and the quaternary ammonium compoundscrystallized, filtered, dried, and weighed. The benzene solution wasthen evaporated, leaving adiponitrile and high boilers. The adiponitrilewas recovered by distillation.

Method II The pH of the organic product from electrolytic cell wasadjusted to 3.54.5 and then filtered. The filter cake was dried andweighed. The organic liquid was extracted with water and methylenedichloride, the water washes being extracted with methylene dichlorideand the methylene dichloride extract being washed with water. Theaqueous layers were consolidated at the end of the extraction, and theorganic layers were separately consolidated.

The water was stripped from the combined aqueous layers, leaving as aproduct dried quaternary ammonium compound.

The combined organic layers were charged slowly through a separatoryfunnel into the distillation apparatus flashing the acrylonitrile and CHC1 off over a steambath. The separatory funnel was then removed, and athermometer was installed where the separatory funnel had been and thedistillation resumed at 40 mm. Hg for about five minutes after visibleboiling had ceased. The pressure was then decreased to 5 mm. Hg, anddistillation continued until a pot temperature of 70 C. was reached. Theadiponitrile was then recovered by distillation.

In the foregoing examples the current efliciency to adiponitrile is thepercentage of the current which is utilized in making adiponitrile.By-products included hydrogen gas, propionitrile, beta-hydroxypropionitrile and beta, beta-oxydipropionitrile, and polyacrylonitrile.The current efficiency can be determined as exemplified by the followingsample calculation. In a run in which the average current is 2.8 ampsand the duration is 156 minutes, the current density is 2.8 15660=26,220 amp-sec. or 0.2720 faraday. If the electrolyte weighs 252grams and has a concentration of 4.27% adiponitrile, the quantity ofadiponitrile is 0.10 mol, and at two faradays per mol of adiponitrilethe current efiiciency to adiponitrile is 0.10/(0.2720 2) which is 65%.

The adiponitrile produced by the disclosed process is useful as anintermediate in the production of nylon.

I claim:

1. A process for the production of adiponitrile which comprises passinga direct electric current at a potential between about 1.2 and 15 voltsand at a cathode current density between about 50 and 200 amp/sq. ft.between a cathode selected from the class consisting of palladium,nickel, chromium on brass, platinum, lead, silver and copper, and aninert anode through an electrolyte consisting essentially ofacrylonitrile, a quaternary ammonium compound and between about 1 andabout 3.8% water, said electrolyte having a pH of less than 6 therebyforming adiponitrile and thereafter recovering the adiponitrile.

2. The process of claim 1 in which the quaternary ammonium compound hasthe formula R N+X where R is an organic radical having 2-8 carbon atomsand X" is a member of the class consisting of Cl, Br, I and R 50 where Ris an alkyl group containing between 1 and 6 carbon atoms.

3. The process of claim 4 in which the quaternary ammonium compound isselected from the class consisting of tetraethyl ammonium bromide,tetrabutyl ammonium iodide, tetrabutyl ammonium bromide, and tetraethylammonium ethyl sulfate.

4. The process of claim 3 in which the anode and the cathode areseparated by a membrane that is cation permselective.

5. The process of claim :4 in which the anode is surrounded by anaqueous acid electrolyte.

6. A process for the production of adiponitrile in an electrolytic cellhaving an anode compartment and a cathode compartment, said anodecompartment being separated from said cathode compartment by means of acationic permselective membrane, which comprises subjectingacrylonitrile consisting essentially of a quaternary ammonium compoundand between about 1 and about 38% water to direct electric current inthe cathode compartment of said cell thereby forming adiponitrile andthereafter recovering the adiponitrile.

7. The process of claim 6 in which the anode compartment of said cellcontains an aqueous acid solution.

8. The process of claim 7 in which the aqueous acid solution is selectedfrom the class consisting of sulfuric, hydrochloric, ethyl sulfuric, andp-toluene sulfonic.

9. A process for the production of adiponitrile in an electrolytic cellhaving an anode compartment and a cathode compartment, said anodecompartment being separated from said cathode compartment by means of acationic permselective membrane which comprises subjecting anelectrolyte to the action of direct electric current, the electrolyte inthe cathode compartment consisting essentially of between about 1 andabout 3.8% water, quaternary ammonium compound, and acrylonitrile,whereby adiponitrile is formed in the cathode compartment of the cell,and recovering adiponitrile.

References Cited UNITED STATES PATENTS 2,726,204 12/1955 Park et al.20472 FOREIGN PATENTS 566,274 1l/ 1958 Canada.

JOHN H. MACK, Primary Examiner H. M. FLOURNOY, Assistant Examiner 3 3UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,'+88,26T Dated January 6 l9'TO Inventor s) WALTER JOHN SLOAN It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

FCLAIM 3, column 5, line 46, "claim 4" should be claim 1 .1

"TIGNED AND SEALED SEP 151970 Amsfing 0mm mm 1:. sammm, JR.

Gomissioner of Patents

